Printing apparatuses (hereinafter referred to as image forming apparatuses) such as printers or digital multifunction peripherals have become widespread. In such an image forming apparatus, an image is formed based on a print job transmitted from a computer terminal, and then a charged photoreceptor is irradiated with light based on the image to form an electrostatic latent image, which is developed with charged toner being attached. The resulting toner image is transferred to a sheet of paper via an intermediate transfer body such as a transfer roller or a transfer belt.
Members such as toners used for an image forming apparatus each have color characteristics. Some colors are easy to reproduce and other colors are difficult to reproduce with them. Therefore, in view of the color characteristics of these members, various types of methods for image forming have been proposed to reproduce the original image color as precisely as possible.
For example, Unexamined Japanese Patent Application Publication No. 2002-182634 discloses an apparatus in which characteristics of a target image are automatically identified and then switching is carried out to an optimum color reproduction processing (color matching) method based on these detected image characteristics. Further, Unexamined Japanese Patent Application Publication Nos. H08-244254 and H10-44473 disclose apparatuses using seven color inks of cyan, magenta, yellow, black (K), red, green, and blue.
In such manners, reproduction methods are switched based on image characteristics, or a color formed via superimposition of plural inks is replaced by a single-color ink, whereby a color close to the original image can be reproduced. However, a toner group containing toners of CMY, CMYK, or CMYK+RGB has a reproducible color area (referred to as a color reproduction area). Therefore, when any color falling outside the color reproduction area is contained in the original image, an image exhibiting a desired color is unable to be reproduced.